Magnetic resonance imaging (MRI) is a non-invasive imaging method that utilizes nuclear magnetic resonance techniques to render images within a patient's body. Typically, MRI systems employ the use of a magnetic coil having a magnetic field strength of between about 0.2 to 3.0 Tesla. During the procedure, the body tissue is also briefly exposed to radio frequency (RF) pulses of electromagnetic energy. The relaxation of proton spins following cessation of the RF pulses can be used to image the body tissue.
During imaging, the electromagnetic radiation produced by the MRI system can be picked up by implantable device leads used in implantable medical devices such as pacemakers or cardiac defibrillators. This energy may be transferred through the lead to the electrode in contact with the tissue, which can cause elevated temperatures at the point of contact. The degree of tissue heating is typically related to factors such as the length of the lead, the conductivity or impedance of the lead, and the surface area of the lead electrodes. The effectiveness of implanted cardiac management devices may be compromised by the heating of cardiac tissue at the lead/heart interface. For example, pacemakers deliver low energy pace pulses that cause the heart to initiate a beat. The minimum voltage of those pace pulses that results in a response from the heart is known as the capture threshold. The capture threshold may increase as a result of localized heating of the lead due to the MRI RF field. Consequently, with an elevated capture threshold for the cardiac tissue, the implantable medical device may not deliver a pulse of sufficient voltage to generate a desired response in the tissue (i.e., loss of capture).